I can't recommend an alternative, but be prepared to wait a long time and to be harassed excessively while waiting in line for a bus. This company totally oversells their tours! For a family of 5, we spent around $300 and now wish we would have just booked a private tour to avoid all of the hassle and annoying sales staff from bus touring companies.
Also, if you purchase the 1-day ticket, be prepared to spend all day waiting for or riding on the bus. I don't know how anyone has time to hop off for stops, especially if you only have one day. That being said, it is a decent way to see a lot of Manhattan, particularly if you don't have many days to explore. Just be prepared to wait and keep your composure!